The present invention relates to a developing device for use in image forming apparatuses such as a copying apparatus, a printer, etc.
When an image is formed by image forming apparatuses such as a copying apparatus, a printer, etc., an electrostatic latent image is initially formed on a surface of an electrostatic latent image support member for supporting the electrostatic latent image, usually a photosensitive member. Then, charged toner is supplied to the surface of the electrostatic latent image support member by a developing device so as to develop the electrostatic latent image into a visual toner image. Subsequently, the obtained toner image is transferred onto a transfer medium such as a paper sheet by a transfer device and is fixed on the transfer medium by a fixing device.
The developing device used in such image forming apparatus is of several types. Particularly, in a developing device in which a thin layer of developer is formed on a developer support member such as a developing roller so as to be brought into contact with the electrostatic latent image support member, it is essential that the thin layer of the developer should be supplied uniformly. A developer regulating blade for forming the thin layer of the developer is required to be brought into contact with a surface of the developing roller at a predetermined pressing force or more. Thus, the developing roller should have a relatively high hardness. Meanwhile, at a contact area between the developing roller and the electrostatic latent image support member, it is desirable that hardness of the developing roller be minimized in order to prevent damage to the electrostatic latent image support member and marring of an image.
In order to solve the above described problem of the known developing device, Japanese Patent Laid-Open Publication No. 63-226676 (1988) proposes a developing device in which at a toner supply area, uniform charging of toner and uniform formation of a thin layer of the toner are improved by securing a sufficiently large pressing force between a toner support member for development and a means is such as a blade for forming the thin layer of the toner, while at an area having an electrostatic latent image support member and the toner support member confronting each other, the toner is supplied to an electrostatic latent image by holding the toner support member in contact with the electrostatic latent image support member at a proper light pressing force stably.
However, also in this prior art developing device, such a case as shown in FIG. 12 may happen in which since a longitudinally central portion SLC of a thin film member SL is curved concavely at the area having the thin film member SL and the electrostatic latent image support member PC confronting each other, it is difficult to uniformly hold the thin film member SL and the electrostatic latent image support member PC in contact with each other. Namely, the cylindrical thin film member SL is generally made of metal, for example, nickel by electroforming or the like. However, the thin film member SL made of metal has such drawbacks that the production is complicated and expensive, the thin film member SL is likely to be cracked at its opposite end portions, the thin film member SL is readily damaged when struck by an object and the thin film member SL is apt to be affected by ozone or the like emitted from a charger of a copying apparatus, etc. In view of these drawbacks of the thin film member SL made of metal, it is proposed that the thin film member SL be made of synthetic resin in place of metal. In the case where the thin film member SL is made of synthetic resin, concave curving of the central portion SLC of the thin film member SL is likely to take place as shown in FIG. 12 more conspicuously than a case in which the thin film member SL is made of metal, thereby resulting in a defective image having skipped spots such as central skipped spots or thin spots. Even if the thin film member SL is made of metal, it is difficult to bring the thin film member SL into contact with the electrostatic latent image support member PC stably and uniformly when thickness of the thin film member SL is small.
Furthermore, the prior art developing device has such problems that the thin film member is required to have high machining accuracy and high assembly accuracy and position of contact of the thin film member with the electrostatic latent image support member is restricted. In addition, if the thin film member is allowed to stand in contact with the electrostatic latent image support member, the thin film member is deformed. As a result, noncontact state may partially appear between the thin film member and the electrostatic latent image support member, thus resulting in production of thin spots or skipped spots in an image.